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Thread: Adding an idle air control (IAC) valve

  1. #1

    Default Adding an idle air control (IAC) valve

    One of the things thatís always bugged me about the Noble is the primitive idle speed control. Every other car on the planet (possibly an exageration) has a means to allow more air into the engine under certain condions, such as cold start or switching on the a/c. But not the Noble. We have to make do with a scheme involving fairly exteme ignition advance and fuel settings to adjust idle speed.

    Evidence suggests that this wasnít always the case. The MBE wiring diagram thatís wandering around the Internet shows an output pin dedicated to driving an idle control valve. And my wiring harness has a wired connector that plugs into the standard Ford valve. Yours probably does too.

    Recently Iíve been playing with a Ford IAC valve purchased from You can find it listed under 2005 Escape. Step one was to simply attach it to the intake, wire it to the ECU, and use the ECU output test function to determine the control range of the valve. These valves are typically controlled by a PWM (pulse width modulated) signal, which is a way of approximating an analog voltage output with a microprocessor by switching digital output pin on and off very quickly. Trial and error suggested that I should use a 1 kHz PWM frequency, and that the control range was only about 70% duty cycle (valve closed) to 85% duty cycle (valve wide open).

    Back to step one. I hooked everything up and fired the cold engine. The valve provided a nice fast idle of about 1300 rpm as indicated by my tuning laptop, but my tach needle was going crazy, swinging all over the dial. I wonít bore you with the electronics theory involved in PWMing an inductive load. (Although I could, if youíre interested! EE was my University major.) Suffice it to say that electrical noise from pulsing the inductive load was getting into the ECU and causing some of the output pins, including the tachometer drive, to pulse in time with the PWM beat.

    This made me wonder if electrical noise was the reason Noble abandoned the IAC valve.

    Step two was to put a flyback diode, sometimes called a freewheeling diode, across the valve connector pins. This resulted in a stable tach needle, but a few drives to work showed that it was not going to be the long term solution. Occasionally, within the first minute after a cold start, the IAC would spontaneously open a nudge, cause the car to surge forward a few feet. Then it would behave normally for the rest of the drive. I had visions of having to sue myself for unintended acceleration!

    Step three was to replace the flyback diode with a zener diode connected between the valve and the engine block. This diode conducts the residual current that flows through the valve coil (that happens when the PWM ground path through the ECU is abruptly opened) to the engine block, rather than recycling it through the coil with way a flyback diode does.

    This did the trick! Now I have a fast idle when the engine is cold, a stable tach needle, and no unintended acceleration. My next step is to fabricate a restrictor plate for the valve so that I can have better control resolution. The valve itself is pretty big in diameter, so a small change in opening results in a fairly large change in rpm. I learned that the valve will take the engine to 5000 rpm, or maybe higher, if you open it all the way. That was an exciting garage moment!

    This mod is very inexpensive and easy to fit. What I donít know is if the MBE output pin is enabled by default, or if the shared version of Easimap can enable it if not. If it is enabled, or can be, it should be very straightforward to set up a fast idle for cold starts and to nudge up idle speed when the a/c kicks in.


  2. #2


    I am at lost, I thought you had changed ECU?

  3. #3


    Nice writeup Jeff! Also, what byl said.

  4. #4


    I did change the ECU, so I have no problem utilizing the valve. I was trying to make clear (unsuccessfully! ) that I was not sure if MBE users would be able to take advantage of this as a DIY mod.


  5. #5


    Now it's clear. You should take french classes to complement you're technical skills set, it would be easier for me to understand

  6. #6
    Join Date
    Jul 2010
    Portland, Oregon
    Rep Power


    Jeff's right that ECU documentation does clearly indicate that IAC valves were considered for our Nobles. I could be wrong, but the extra plug that's included on our harnesses appears to be for an IAC. EasiMap 5.0 that is available to all of us also seems to indicate that the ECU can be programmed to control an IAC. I haven't tried to install and use one, though. My engine idles pretty well w/o one, but it might be fun to try one someday.
    2007 M400, NUS 206

  7. #7


    Quote Originally Posted by byl View Post
    Now it's clear. You should take french classes to complement you're technical skills set, it would be easier for me to understand
    I took French as an elective towards my bachelor's degree and German in high school, but that was several month ago. Or maybe decades. I forget.


  8. #8


    Quote Originally Posted by Bill View Post
    (snip) the extra plug that's included on our harnesses appears to be for an IAC.
    It is. Mine is connected now.


  9. #9


    Update. The idle air valve experiment has not been an unqualified success. The valve I bought does not respond the way I expected it to. The biggest gripe is that it does not really close. If I disconnect the wires, it leaks enough air to run the engine at about 1200 rpm with the throttle blade as shut as it goes.

    I did more digging and learned from a follow DIY'er in the UK that an earlier model year valve was working well. Silly me, I went with the most recent part (2005 Escape) thinking that it would have all the latest design refinements. I ordered a second valve from, this time for an earlier year. This valve is normally open, so the solenoid coil drives it closed, not a too-weak spring. The valve itself, which is a different design and diameter, seems to seal better. The whole thing seems to be more robustly constructed.

    I'll toss it on this weekend, test the control range, and hope that I can have a nicely tuned hot and cold running idles.


  10. #10
    Join Date
    Jan 2009
    Rep Power


    I have fitted the idel control valave as well... with the electrics disconnected the idle is the same as when the blanking plug was fitted... with power across the valve it revs (but not to 5000rpm!) to over 2000rpm.

    I've not yet refined it because I have not yet been able to establish the parameters that the valve requires... do you have any details about the PWM settup?

    Im also thinking I may use this to slow the revs drop between gearchanges so it doesn't drop like a rock when changing down.

    edited to add that my valve is open by default... the vacume of the engine closes it rather than it needing power... so if you disconect the power it will be sucked closed on the car and idle as though its blanked off.
    Last edited by andygtt; 01-25-2013 at 01:35 AM.

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